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Biology 2483A
Mark Moscicki

Ecology-Lecture 9 Oct 10 2013 Case Study-Human Population Growth  Humans have a large impact on the global environment Figure 9.2 Explosive Growth of the Human Population for two reasons. Our populations have grown explosively, along with our use of energy and resources  Human population reached 6.8 billion in 2010, more than double the number of people in 1960 (addition of nearly 4 billion)  Our use of energy and resources has grown even more rapidly. From 1860 to 1991, human population quadrupled in size, and energy consumption increased 93 fold  For thousands of years, our population grew relatively slowly, reaching 1 billion for the first time in 1825. Now we are adding 1 billion people every 13 years  Growth rate has slowed recently, to about 1.18% per year, and continues to slow  By 2080, it is predicted that there will be roughly 9-10 billion people on earth Carrying Capacity  Is 10 billion above the carrying capacity of the human population?  Many people have tried to estimate human carrying capacity  Researchers must make assumptions about how people would live and how technology would influence our future  Estimates range from fewer than 1 billion to more than 1,000 billion Ecological Footprint  Total area of productive ecosystems required to support a population  Uses data on agricultural productivity, production of goods, resource use, population size and pollution  The areas required to support these activities is then estimated  Stats for 2006  11.9 billion hectares of productive land available globally  Average ecological footprint: 2.6 hectares  Suggests a carrying capacity of 4.6 billion  Population: 6.6 billion, a 40% overshoot of carrying capacity  If everyone in the world used resources at the same rate as:  U.S citizens in 2006, carrying capacity would be 1.3 billion people  Indian citizens in 2006. carrying capacity would be 14 billion people Introduction  One of the ecological maxims says "no population can increase in size forever  The limits imposed by a finite planet restrict a feature of all species: A capacity for rapid population growth  Ecologists try to understand the factors that limit or promote population growth Life Table  Summary of how survival and Table 9.1 reproductive ages vary with age  Information about births and deaths is essential to understand current population trends and predict future population size  Life table data for the grass Poa Annua were collected by marking 843 naturally germinating seedlings and then following their fates over time. This is a cohort life table because it shows the fate of a group of individuals born during the same time frame.  Sx=survival rate: Chance that an individual of age X will survive to age X+1. This is calculated from Nx  Ix=survivorship: Proportion of individuals that survive from birth to age X. This is calculated from Nx  Fx=fecundity: Average number of offspring a female will have at age X  Birth and death rates can vary greatly between individuals of different ages  Gambians survivorship depends on the season of birth  Gambians born during the "hungry season" (when food stored from the previous year is depleted) had lower survivorship than those born at other times of the year  Also note that survivorship can vary depending on where you live. United States survivorship is much higher (late 70s) than Gambian survivorship (40s)  In some species, age is not important. For example, in some plants reproduction is more dependent on size (related to growth conditions) than age. Therefore, life tables can also be based on size or life cycle stage Survivorship Curve  Plot of the number of individuals from a hypothetical cohort that will survive to reach different ages. Survivorship data from life tables can be graphed as a survivorship curve  Survivorship curves can be classified into 3 types which indicate life stages at which high rates of mortality are most likely to occur  Type 1: Most individuals survive to old age (Dall sheep, humans) Newborns, juveniles, and young adults all have high survival rates. Death rates do not begin to decrease until old age  Type 2: The chance of surviving remains constant throughout the lifetime (some birds)  Type 3: High death rates for young, those that reach adulthood survive well (species that produce a lot of offspring) These are the most common type observed in nature. Populations  A population can be characterized by its age structure (proportion of population in each age class)  Age structure influences how fast a population will grow  If there are many people of reproductive age (15-30), it will grow rapidly  A population with many people older than 55 will grow more slowly  Life table data can be used to predict age structure and population size  If the population starts with 100 individuals:  Age class 0:(n0)= 20 individuals  Age class 1:(n1)= 30  Age class 2:(n2)= 50  To predict population size for the following year, calculate:  # of individuals that will survive to the next time period. To calculate this, multiply the number of individuals in each age class by the survival rate for that age class  # of offspring those survivors will produce in the next time period Growth Rate  If survival or fecundity rates change, population growth rate will change. Age distribution will also change.  The growth rate will remain constant when the population and each of its age classes increases by a constant multiplier each year. The proportion of individuals in each age class will also remain the same.  Ex: If F1 increases (and other values stay the same), growth rate decreases  Environment factors can alter survival or fecundity and thus change population growth rates. Knowledge of these factors helps develop management practices to decrease pest populations or increase an endangered population  Birth and death rates can be affected by a broad range of abiotic/biotic factors
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